[libc++] Make __dir_stream visibility declaration consistent

The class `__dir_stream` is currently declared in two places: as a
top-level forward declaration in `directory_iterator.h`, and as a friend
declaration in class `directory_entry`, in `directory_entry.h`.

The former has a `_LIBCPP_HIDDEN` attribute, but the latter does not,
causing the Firefox build to complain about the visibility not matching
the previous declaration. This is because Firefox plays games with
pushing and popping visibility.

Work around this by making both `__dir_stream` declarations consistently
use `_LIBCPP_HIDDEN`.

Reviewed By: ldionne, philnik, #libc

Differential Revision: https://reviews.llvm.org/D121639